Realistically, it would be common practice for DG agents to use fake IDs, especially if they come from agencies that seem a rather odd combination for a given case, or for any investigative field work, say an INS agent, someone from the CDC, and an Air Force OSI agent investigating the disappearance of children in an area nowhere near an Air Force Base, where no disease was in evidence. Even not very bright NPCs might suspect something unless the agents identify themselves falsely, in this case probably as three agents of the FBI. (Marco)

People questioned by agents with odd affiliations might call one or more of the agencies, and ask if the investigators are legit. Unless under deep “Bureau Special”-type cover, their non-DG supervisors will be puzzled, and internal affairs might look into their doings. (Marco)

Ideally, witnesses will not get close enough or have enough time to discern a badge number or other traceable data. (Phil Ward)

DG agents won't normally move as a group to interview harmless people, since it is faster and safer to spread out. Unless working alone, they operate in pairs, with only one agent showing his or her badge, and introducing the other one. One agent asks the questions; the other takes notes. If the interview is performed correctly, witnesses may hardly even notice the second agent, so his or her affiliation won't matter. (Phil Ward)

Friendlies may be involved in extensive on-site questioning. They might be locals or people with legitimate interests in the 'incident', and do not have to know much or anything about DG. These “puppets” will be less noticeable than agents, especially agents without obvious jurisdiction. Remember, DG agents will realistically come across 'problems' and false leads in the course of their legitimate investigations before they find any evidence of 'occult involvement'. This type of activity is just as important to the cause as dealing with paranormal threats, and should be pursued just as vigorously. It is especially useful when DG agents have a hard time getting away from their regular work. (Phil Ward)

Friends of friendlies are cumbersome to administrate but can be a helpful addition to this idea. These are contacts the friendlies have, but who don't have any contact with DG agents at all, or no more than very brief contact. In need, the DG agents can start using them directly, at which point they become friendlies. (David Farnell)

Other territorial law enforcement officers might look into things if their own jurisdiction seems threatened. In the example above, the case may really be appropriate for the FBI, so it may be necessary for Adam to prevent sending a real team. (Marco)

To reduce this risk, non-specialist DG agents are likely to operate in their own area, where this might make them appear more suitable than contending agencies. As agents begin to specialize in a particular type of activity, they are going to be called out of their home turf and become involved in other investigations which are less and less plausible, hence more likely to happen under false affiliations lest the agents are prosecuted. (Phil Ward)

DG agents will typically have to fend for themselves in explaining their presence and justifying the dominance of their alleged affiliations in an investigation. Difficult cover-up work from the highest levels of DG, to prevent all such conflicts and get exclusivity, is reserved for very important missions. (Phil Ward)

With a major agency overlap, exclusivity may be impossible. Say, for example, a secret militia cult was smuggling in high-power assault weapons, then the case might involve agents from BATF, the FBI, and, of course, local authorities. Without exclusivity DG may be able to plant a few agents in the inter-agency task force, but won't be able to dominate it. In this case, a BATF DG friendly may be given instructions on how to recognize the involved DG agents, perhaps through a secret code. (Christian Conkle)

Problems with local law enforcement officers are an obstacle to our cause, regardless of federal affiliations. It is recommended to cultivate good relations with any law enforcement representative who may have bearing on a current case. Providing them with credit for arrests, forensic data they may otherwise have to wait a long time for, and plausible explanations for any weird occurences they may find will all go a long way to keeping local LEOs out of our business. (Phil Ward)

Other covert organizations might catch wind eventually if the same odd trio repeatedly cropped up at or near weird phenomena. (Marco)

If such organizations have power within the real agencies of the DG operatives, this is a greater threat. (Andrew)

Many other people will complicate covert investigations. DAs who require simple and scandalous answers. Mayors who need quick results so they can be “tough on crime”. Civil rights activists, and so forth. Talk shows, supermarket tabloids etc. will be more likely to produce DG investigations than to halt them. (Phil Ward)

On obtaining and designing fake identities:

A major function of friendlies may be to acquire fake IDs and manage security trails, responding to departmental inquiries etc. (William Timmins)

Fake IDs are especially useful if they can be tied to someone completely different. IDs seized in other (legitimate) investigations are great, if removed from evidence lock-ups without trace. (Phil Ward)

Each agent may have a temporary re-assignment or cover identity based on their code name; for example, Agent Quentin becomes Quentin Hall of the agency of the week. This reduces confusion. (Daniel Harms)

The CIA manufactures fake IDs constantly to support the sanctioned and unsanctioned missions of the Operations Directorate, complete with establishing correlated identities for the case officer's background references, past neighbors, past employers, and so forth, calling on other case officers to play these roles and routing the appropriate "local" phone numbers to them. And the CIA is hardly the only intelligence agency around which must equip its operatives with credible false histories. Delta Green is positioned to call on such resources to support its own black operations without compromising its security: the random CIA officer who is posing as a DG agent's past employer, for example, does not need to know anything about the DG agent in question except what is necessary to describe his or her false work history. (Shane Ivey)

Avoid a fake identity that, when uncovered, just reveals who you are, and are not. Have another layer of identification ready. If your opponents work hard enough, they may believe they've really found you out and will look no further, averting harm to Delta Green. (David Farnell)

Fake identities are especially important when DG agents work closely with friendlies, doubly so when the investigation deals with entities suspected of controlling minds. Friendlies who may have been compromised by such entities can be excommunicated with little risk if fake identities are used. (Andrew)

Keep in mind that agents recruited into DG after getting to know friendlies or enemies, which is the typical case, won't be protected from them by fake IDs. Experienced agents may also encounter old or new threats while working their day jobs. (Christian Conkle)